Pain sensitivity of primary teeth as compared to permanent teeth is an area of clinical concern to the dentist. The first part of this study will approach the problem by determining the number of myelinated and unmyelinated nerves entering human teeth and cumulative circumferences of myelinated axons. Electron microscopy will be used for accurate identification of unmyelinated nerves and for accurate measurement of axons. Numerous electron micrographs will be assembled to form montages of juxta apical cross sections. Identification and measurements will be made from the montages. It is thought that a determination of the number of nerves and amount of nervous tissue entering the tooth will give an indication of the amount of sensory impulse which the tooth is capable of passing along. Statistical comparisons of 20 fully developed human primary and 15 permanent teeth from adult humans will be made. It is felt that this sample size will be satisfactory for the present investigation. One of the important points of this study is to determine whether there is a large difference in innervation levels between primary and permanent teeth. A large difference can be demonstrated statistically. If the differences are small, a significant clinical difference in sensitivity would not be suspected from this study. Ratios of myelinated to unmyelinated nerves will be compared for primary and permanent teeth. This may give some insight into the predominant type of impulses for primary and permanent teeth, since it is thought that myelinated and unmyelinated nerves sensory for pain transmit different types of impulses. Pain sensitivity of primary teeth at different stages of root resorption is of equal concern. In the second part of the study, correlation of root resorption amount with percent of nerves showing degeneration will be done. This should give some indication of tooth sensitivity at various stages of root resorption.